Green roof runoff : an innovative method for wintry monitoring

Since late 1990’s, storm water management has required the Best Management Practices (BMPs) implementation to mitigate urbanization drawbacks. Green roof is one of a series of available BMPs, so more and more green roofs are used in cities to restore the urban hydrological balance. High level researches have been conducted since then to better understand the hydrological efficiency of green roof in temperate climates. However, that information lacks in cold climates and very few studies deal with green roof behavior in winter. The new building of Polytechnic Montreal in Quebec Canada was built in 2005, and a 900 m² semi-intensive green roof (Hydrotech®) was planted on it which contributed to its LEED® certification. The aim of this study is to measure runoff quantities from the Polytechnic green roof. A new instrument, called drain flow meter (DFM), was designed to measure storm water runoff flowing through a drainage downpipe. It is composed of a 60° V-notch weir and an ultrasonic sensor installed in a cylindrical container that have a diameter of 85 mm. The DFM has been installed in a drainage down pipe draining a green roof section of 65m². The DFM water level was logged every 5 minutes starting in October 2013 and finishing in January 2014. Laboratory calibration shows a good precision for small flows (<20mL/s), a quite good precision for medium flows (<150mL/s) and a poor precision for high flows (>150mL/s). Preliminary results are promising concerning peak flow delay measurement in fall and in winter. This study shows that the hydrological monitoring of a green roof in winter is possible. The DFM will be of particular interest to practitioners who wish to improve the efficiency of green roof in winter.

More DFMs should be implemented to compare runoff between the bare roof and the green roof and to assess green roof retention capacity. Also, a snow gauge could be used to adequately measure snowpack on the green roof in winter.